In a file sharing system in which a file is shared among a plurality of users after a user uploads the file to a server, a method of re-encryption is known to maintain secrecy of file with respect to the server.
A conventional re-encryption scheme presents no particular problem, but if a problem should arise, a server is enabled to create re-encrypted text without the permission of each user. Therefore, from the viewpoint of realizing safe file sharing, there is room for improvements in a proxy re-encryption scheme.